crystal info

[perts]

i am desinging a circuit with a freescale 9s12 mcu. I have a crystal that i want to use to provide the 8mhz clock.

i wanted to know how to calculate the caps (C) on the crystal to get the proper crystal setup.

i know you need to worry about trace capacitance and capacitance from the actual micro

the specs for the crystal i have is
load cap: 8pf
esr: 220 ohms
drive level: 10 micro Watts (500 micro Watts)


| |
| _ |
| | | | | |
-----| | | |-----
| | |_| | |
| |
--- ---
--- C --- C
| |
| |
gnd gnd

 

[MacGyverS2000]

The caps are in parallel (the load to the crystal), so 16pF each is the straight out answer. Expect a couple of pF stray capacitance on the traces themselves. This puts you in the right ballpark and shoudl work for almost every case. However, depending upon environment, you may need to tweak one cap value or the other to increase/decrease start-up time, etc.

Do a search on this board... I posted some useful theoretical and practical tips calculating crystal cap values and other such stuff just a few months ago.

 

[MacGyverS2000]

Found it...

http://www.eng-tips.com/viewthread.cfm?qid=94043

 

[logbook]

Two 16pF capacitors in SERIES gives the 8pF load capacitance, doesn’t it?

I agree with the 16pF capacitors, as stated, it is just that they are effectively in series with each other, the series connected pair being in parallel with the crystal.

 

[MacGyverS2000]

^
|___ What he said...